Blend together butter, brown sugar, vanilla, and salt in a bowl
with a fork. Sift in flour and blend with fork until a soft dough forms.

Spread dough evenly in baking pan using an offset spatula or back of a spoon, then prick all over with fork.

Bake until golden, 15 to 20 minutes, then cool completely in pan on a rack, about 30 minutes.

Make chocolate dulce de leche:
Bring cream and dulce de leche
to a simmer in a small heavy saucepan, stirring with a wooden spoon until dulce de leche has dissolved.

Whisk together yolks in a bowl, then slowly whisk in hot cream mixture. Return to pan and cook over medium heat, stirring constantly, until pan is visible in tracks of spoon and mixture registers 170°F on an instant-read thermometer. Remove
from heat and whisk in chocolate until melted.

Make bars:
Pour chocolate mixture over cooled shortbread and chill, uncovered, until cold and set, about 2 hours.

Run a small knife around edges to loosen, then transfer to a cutting board using parchment. Cut with a hot clean knife
(dip in hot water and wipe clean between cuts) into 24 bars. Chill until ready to serve.

Cooks' note:
Chocolate dulce de leche bars can be chilled in an airtight container up to 1 day.